GHH Nervous Tissue

General Histology and Histotechnique (1st semester; 2012-2013)

NERVOUS TISSUE Nervous system - Complex, highly organized network of billions of

neurons and even more neuroglia Components: Brain – contain 100 billion of neurons Cranial nerves and their branches - 12 pairs of left and right, emerge from the base of

brain. Spinal cord - It connects to the brain through the foramen

magnum - Spinal nerves – 31 pairs emerging from the spinal

cord. Spinal nerves and their branches Ganglia – small mass of nervous tissue and contains

cell bodies of neurons Location: outside of central nervous tissue, outside of the brain & spinal cord - Closely associated with the cranial nerve and spinal

cord. Enteric plexus – located on the walls of digestive

tracts – G.I.T. Function: Regulate the functions of Digestive system Sensory receptors - Either dendrites of sensory neurons - Specialized cells that monitor change in the internal

wall as well as the external environment. Ex. Pacinian corpuscles, ductile corpuscles Major structures of Nervous system Three Basic functions of Nervous System 1. Sensory function - Internal: increase in blood acidity - External: raindrop landing on arm Sensory or afferent neurons - Neurons that carry information into the brain and

spinal cord. 2. Integrative function - Integrates (process) sensory information - Analyze, store, made decisions regarding

appropriate responses Interneurons: neurons for integration - Makes up vast majority of neurons in the body 3. Motor function

- respond and integrates decision Motor or efferent neurons - Carry information out of the brain and spinal cord

going to the effectors Effectors - Cells and organs innervated by motor neurons - Simply the muscle and glands. Nervous Tissue - Responsible for communication and sending

impulses. Consists of Two principal kinds of cells: 1. Neurons/ nerve cells 2. Neuroglia/ glial cells I. Neurons - Responsible for most special functions attributes to

the nervous system: o Sensing o Thinking o Remembering o Controlling muscle activity o Regulating glandular secretions

- Have the property of electrical excitability Capacity to produce

action potential or responses to stimuli

- Many neurons also contain Lipofuscin Pigments Occur as clumps of

yellowish brown granules in the cytoplasm of neurons.

Product of neuronal lysosomes that accumulate as neuron ages.

Parts: 1. Cell body - Star shaped Components: o Cytoplasm = Neuroplasm o Plasma membrane = Neurolemma

Organelles: o Mitochondria o Lysosomes o Golgi complex o Nissl bodies


- prominent clusters of rough endoplasmic reticulum Function: - Produces proteins that are used by the cell body - Replaced cellular components - Used as a material for growth of neurons - Regenerate damage axons in PNS 2. Processes a. Dendrites (little trees) - receiving input portion of the neuron b. Axon (axis) - Propagates nerve impulses toward another neuron,

a muscle fiber, or a gland cell. - A long, thin, cylindrical projection Parts: 1. Axon Hillock - Resemble a small hill - Cone- shaped elongation that joins the cell body

and the axon. 2. Initial segment - First part of axon *** 1 &2 Function: - Responsible for the occurrence of impulses in an

axon known as trigger zone. 3. Axoplasm – cytoplasm of axon 4. Axolemma – plasma membrane that covers the

axon.

5. Axon Collateral - Side branches of axons at right angle of the axon Function: Provides modulation of axon/cell firing 6. Node of Ranvier - Neurofibril node - Indentation between the axon connecting one from

the other. Importance: To speed-up the process of transmitting signals from the CNS to effectors cells (Muscle cell glands). 7. Axon terminal - Fine processes at end of the axon - Swell into bulb shaped structures - Synaptic end bulb

Synapse - the site of communication between two neurons or

between a neuron and an effectors cell Synaptic end bulb: bulb-shaped structure of axon terminals. Varicosities: String of swollen bumps of axon terminals - Contains synaptic vesicles

Stores chemical neurotransmitter Triggers acetylcholine

- Neurotransmitter - acetylcholine Influenced the activity of other

neurons, muscle fibers and gland cells. Types of transport systems: 1. Slow axonal transport - Slower system - Moves materials about 1-5mm per day - Conveys axoplasm in one direction only - From cell body to axon terminals - Supplies new axoplasm for developing or

regenerating axons - Replenishes axoplasm in growing and mature axon - Carry materials from body to axon then back again 2. Fast axonal transport - Faster system - Capable of moving materials a distance of 200-400

mm per day - Uses proteins that function as motors to move

materials in both directions – away from and toward the cell body – along the surface of microtubules

- Moves various organelles and materials that form the membranes of the axolemma, synaptic and bulbs and synaptic vesicles

Clinical application Tetanus - Fast axonal transport is the route by which some

toxins and disease – causing viruses make their way from axon terminals near the skin cuts to cell bodies of neurons, where they can cause damage.

- Toxin produced by Clostridium tetanus is carried by fast axonal transport to the CNS disrupting the actions of motor neurons, causing prolong painful muscle spasms – condition called Tetanus.

Structural Diversity of Neurons


Neurons display great diversity in size and shape A. According to function

1. Motor neuron - Transmit impulses to the motor cells 2. Sensory neuron - Transmit impulses from neurons to the senses 3. Associative neuron - Neurons connecting motor from sensory neuron B. According to number of nerve process 1. Multipolar neurons - Have a several dendrites and one axon 2. Bipolar neurons - One dendrites and One Axon Location: Retina of the eye, inner ear, olfactory area of the brain. 3. Unipolar neurons - Sensory neurons that originate from the embryo as

bipolar - One dendrite and One axon C. According to size and shape 1. Interneurons - Occupy 90% of the total neurons of the body Function: Carry impulses from one neuron to another a. Pyramidal cells - Pyramid like shape - Center of cerebral cortex

Largest part of brain b. Purkinje cells - Found in the cerebellum c. Renshaw cells/ Inhibitory interneuron - Stellate cells - Found in the spinal cord II. Neuroglia - Constitute about half the volume of CNS - 5-50 time more numerous than neurons - Do not generate action potential

- Multiply and divide in the mature nervous system - Supporting tissues of the brain and spinal cord Types: 1. Astrocytes - Largest neuroglia - Star shape with long processes Functions: - Help maintain appropriate chemical environment

for the generation of action potential - Provides nutrients to neurons - It takes up excess neurotransmitters and participate

in the metabolism of neurotransmitters Two types of Astrocytes: a. Protoplasmic astrocyte

- Abundance of cyplasmic granules - Found in gray matter b. Fibrous astrocyte

- Long-slender - Found in white matter 2. Oligodendrocyte - Round/oval body - Few processes - Smaller than astrocyte Functions: - It forms the supporting network around the CNS

neuron - Produce myelin sheath around axon of CNS neurons 3. Microglia - Small cells - Fewer than oligodendrocyte - Derived in mesodermal cells - It gives rise to monocyte and macrophages Functions: - It protects CNS cells from diseases - Engulfed microorganisms - Clear away debris/dead cells 4. Ependymal cells - Epithelial cells and arranged in single layer with

microvilli and cilia - Shape: Columnar cuboidal - Ciliated Functions:


- It lines the ventricles of brain and central canal of spinal cord

- It forms the CSF (cerebrospinal fluid); assist in circulation

5. Schwann cells - Flattened cell - Encircles the PNS axon - Produces myelin sheath around the single axon of

PNS neuron Function: - Participate in the regeneration of PNS axon. 6. Satellite cell - Surrounds neurons cell body Function: - Support neurons in ganglia Nerve fibers - General term for any process (axon of dendrite)

projecting from the cell body of a neuron - It is also a connective tissue Connective tissue sheaths: 1. Epineurium - Encircles the entire nerve 2. Perineurium - Wrapped around individual fascicles *** 1&2 – composed of dense connective tissue 3. Endoneurium - Surrounds the individual axon - Loose Areolar connective tissue Types of Nerve fibers: 1. Myelinated nerve fiber - With myelin Nodes of Ranvier – gaps in the myelin sheath Internodes Significance of myelin sheath:

1. Increase speed of nerve impulse transmission due to node of ranvier by salutatory conduction: electrical alteration jumps as current flow from one node of ranvier to another

2. Electrically insulates the axon of neuron

3. Gives color to the white matter in the CNS 2. Unmyelinated nerve fiber - No myelin sheath - Surrounded by coat of neuroglial plasma membrane - Fiber is enclosed by the Schwann cell only in PNS - Fiber is not unsheathed in CNS - Impulse is conducted as a continuous wave of

voltage reversal to the end of the axon. Neuroglial cells that produces Myelin sheath: 1. Schwann cells in PNS - Begin to form myelin sheath around axon during

fetal development neurolemma: sheath of Schwann enclosing myelin sheath.

- Aids in regeneration that forms regeneration tube that guides and stimulates regrowth of axon.

Nodes of Ranvier: gaps in myelin sheath 2. Oligodendrocytes in CNS - Myelinated parts of many axons in the same

manner that a Schwann cell myelinates part of a single PNS axon

- Neurolemma is not present - Nodes of Ranvier present but few in number - Axon in CNS display little regrowth after injury due

to absence of neurolemma - Certain diseases such as multiple sclerosis and Tay-

sachs diseases destroy myelin sheath Gray and White Matter White matter - Aggregations of Myelinated processes from many

neurons - In spinal cord, white matter surrounds an inner core

of gray matter shaped like a butterfly or the letter H.

Gray Matter - Contains neuronal bodies, dendrites, unmyelinated

axons, axon terminals, and neuroglia - Looks grayish rather that white because there is

little or no myelin in these areas. - In the brain, a shell gray matter covers the surface

of the largest portions of the brain, the cerebrum and cerebellum.

Education

GHH Nervous Tissue